Peroxide-curable fluoroelastomers and chlorofluoroelastomers having bromine and iodine curesites and the preparation thereof

ABSTRACT

Peroxide-curable fluoroelastomers and chlorofluoroelastomers are prepared in the presence of iodinated compounds, with iodine bonding to a substantial number of terminal positions on the fluoroelastomer, and comprising up to 3 weight percent of bromine-containing units selected from fluoroolefins and perfluoroalkyl perfluorovinyl ethers, and complementally at least 97 weight percent of either units of vinylidene fluoride, one or more fluoroolefins, and optionally perfluoroalkyl perfluorovinyl ethers, or units of tetrafluoroethylene, perfluoroalkyl perfluorovinyl ether, and ethylene. The resulting fluoroelastomers have improved processability, excellent strength and compression set properties, and are useful in the preparation of injection molded shaft seals, gaskets and other molded parts. The fluoroelastomer is prepared by a continuous emulsion polymerization process.

FIELD OF THE INVENTION

This invention relates to fluoroelastomers having enhanced physicalproperties and to processes for preparing such fluoroelastomers. Moreparticularly, this invention relates to fluoroelastomers andchlorofluoroelastomers containing bromine that are polymerized in thepresence of iodinated compounds to form curesites at random points alongthe polymer chain and at the ends of the chains, and to continuousprocesses for preparing such elastomers.

Vinylidene fluoride-based fluoroelastomers such as copolymers ofvinylidene fluoride with hexafluoropropylene, optionally withtetrafluoroethylene, have achieved outstanding commercial success, andmay be crosslinked by use of bisphenol curing systems such as aredescribed in U.S. Pat. No. 3,876,654.

Curing systems were subsequently developed that could crosslinkfluoroelastomers with higher concentrations of tetrafluoroethylene andcorrespondingly lower concentrations of vinylidene fluoride thanpreviously treatable. In systems described in U.S. Pat. Nos. 4,035,565and 4,214,060, fluoroelastomer copolymers including bromine-containingfluoromonomer cure in the presence of a radical-generating peroxide anda crosslinking coagent, such as triallylisocyanurate. The presentfluoropolymer represents an improvement over these references in that ithas improved processibility and physical properties where crosslinked byperoxides.

Fluoroelastomers that contain iodo groups on the chain ends have beenprepared, as described in U.S. Pat. No. 4,243,770, by semi-batchpolymerizations carried out in the presence of iodine-containingfluorocarbon or a chlorofluorocarbon chain transfer agent. When thechain transfer agent contains two iodo groups, and when thepolymerization is carried out under suitable conditions, most of thefluoroelastomer chains contain iodo groups at each end, and suchpolymers when treated with peroxide curing agents and a crosslinkingcoagent form a network by linking up of the chain ends. U.S. Pat. No.4,243,770 also teaches the use of a copolymerizable iodo-containingfluoromonomer but since extensive chain transfer occurs at the iodosite, this monomer behaves as a branching site, and at highconcentration gives gelled, difficultly processible fluoroelastomer.

The semi-batch polymerization taught by U.S. Pat. No. 4,243,770 isinherently slow. Moreover, when continuous, high-productivity emulsioncopolymerization in the presence of iodo chain transfer agents iscarried out, chain transfer by iodine is inefficient, so that not allchains have iodo groups at both ends, and vulcanizates prepared byperoxide cure have poor properties. In contrast, the presentfluoropolymer provides a product with excellent properties that isprepared in a continuous emulsion polymerization process.

It is an object of this invention to provide fluoroelastomers that reactwith peroxide curing agents and crosslinking coagents to give uniquepolymer networks in which crosslinks are formed both at random pointsalong the polymer chain and at the ends of the chains. It is anadvantage of the present invention to provide polymers having excellentstrength and compression set properties, as well as good processingcharacteristics. It is a further object of this invention to provide acontinuous, high productivity process for the preparation of saidperoxide-curable polymer. These and other objects, features andadvantages of the invention will become apparent in the description ofthe invention that appears below.

SUMMARY OF THE INVENTION

The present invention provides a peroxide-curable fluoroelastomer,prepared by continuous emulsion polymerization in the presence ofiodinated compounds of the formula RI_(n), where R is a radical of 1-8carbon atoms and selected from the group consisting of fluorocarbons andchlorofluorocarbons, I is iodine, and n is 1 or 2. The iodine is bondedat a substantial number of terminal positions on the fluoroelastomer.The amount of iodinated compound is sufficient to provide at least 0.1weight percent iodine in the fluoroelastomer. The composition comprises

(a) up to 3 weight percent, based on the total weight of the components(a) and (b) of polymer repeat units selected from the group consistingof fluoroolefins and fluorovinyl ethers, said units containing bromineand being present in an amount sufficient to provide 0.1-1.0 weightpercent bromine in the fluoroelastomer, and

(b) complementally, at least 97 weight percent, based on the totalweight of components (a) and (b), of:

(1) polymer repeat units of vinylidene fluoride and polymer repeat unitsof one or more fluoroolefins copolymerizable therewith, saidfluoroolefins containing 2-8 carbon atoms and at least as many fluorineatoms as carbon atoms, and optionally, polymer repeat units provided byperfluoroalkyl perfluorovinyl ethers, or

(2) 32-60 mole percent of polymer repeat units of tetrafluoroethylene,20-40 mole percent of polymer repeat units of perfluoroalkylperfluorovinyl ether and 10-40 mole percent of polymer repeat units ofethylene.

According to another embodiment of this invention, there is provided acontinuous emulsion polymerization process for preparing aperoxide-curable fluoroelastomer of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention there is provided a peroxide-curablefluoroelastomer containing 0.1-1.0 weight percent, preferably 0.1-0.5weight percent iodine, and having polymer repeat units ofbromine-containing comonomer constituents, located in recurring randomlocations along the polymer chain, such that the polymer will contain0.1-1.0 weight percent, preferably 0.15-0.6 weight percent bromine. Anespecially preferred component (a) is4-bromo-3,3,4,4-tetrafluorobutene-1 hereinafter called BTFB. In additionto bromine cure sites located randomly along the fluoroelastomer chain,the invention includes iodine crosslinking sites located at terminalpositions on the polymer chain. This can be accomplished by conductingthe radical copolymerization of the above named monomers in the presenceof an iodinated compound represented by RI_(n), where R is afluorocarbon or chlorofluorocarbon radical of 1-8 carbon atoms, I isiodine, and n is 1 or 2. In the course of the radically initiatedcopolymerization the iodinated compound acts as a chain transfer agent,resulting in a telomerization polymerization process in which a labile,iodine-containing chain end is formed, and the residue of the iodinatedcompound is attached to the other end of the polymer chain. If theiodinated compound has two iodine groups, the fluoroelastomer chain maytherefore have iodine groups at each end. Examples of suitable RI_(n)compounds are perhalogenated diiodides such as1,3-diiodoperfluoro-n-propane, 1,4-diiodoperfluoro-n-butane,1,3-diiodo-2-chloroperfluoro-n-propane, and1,5-diiodo-2,4-dichloro-perfluoro-n-pentane and others known in the art.1,4-Diiodoperfluoro-n-butane is especially preferred.

The amount of iodinated compound that will be employed is high enough togive extensive chain transfer and incorporation of a substantial levelof iodine end groups. High chain transfer efficiency by the iodinatedcompound results in a fluoroelastomer with lower compound viscosity anda relatively narrow molecular weight distribution for desirable rheologyand processing characteristics.

The concentration of iodine in the fluoroelastomer will depend upon theconcentration of RI_(n) in the polymerization medium and uponpolymerization conditions, which will affect the chain transferefficiency. The lower limit of iodine content in the fluoroelastomer isapproximately that at which an effect on peroxide cure rate andvulcanizate properties is found. The upper limit of iodine content inthe fluoroelastomer corresponds approximately to the practical lowerlimit on polymer viscosity, since higher concentrations of RI_(n) givepolymers with lower molecular weight and viscosity. The upper limit oniodine content also relates to the desired highest state of cure.

The polymers of this invention will contain bromine curesites introducedby the bromine-containing units of component (a) of the fluoroelastomer.These units may be a bromine-containing olefin, containing anotherhalogen, preferably fluorine. Examples are bromotrifluoroethylene,4-bromo-3,3,4,4-tetrafluorobutene-1 and a number of others noted in U.S.Pat. No. 4,035,565, previously cited, and these are incorporated hereinby reference. Brominated fluorovinyl ethers useful in the inventioninclude CF₂ Br--Rf--O--CF═CF₂, such as CF₂ BrCF₂ OCF═CF₂, cited in U.S.Pat. No. 4,745,165 and of the type ROCF═CFBr or ROCBr═CF₂ where R is alower alkyl group or fluoroalkyl group, such as CH₃ OCF═CFBr or CF₃ CH₂OCF═CFBr, cited in U.S. Pat. No. 4,564,662. The choice ofbromine-containing units is based on ease of copolymerizability with themajor monomers and low branching tendency, in addition to cost andavailability.

Several useful embodiments of the present invention differ with respectto the composition of component (b)(1) of the fluoroelastomer. One suchcomposition contains polymer repeat units of vinylidene fluoride andpolymer repeat units of either hexafluoropropylene orpentafluoropropylene. In another composition, the component (b)(1)includes polymer repeat units of vinylidene fluoride, polymer repeatunits of tetrafluoroethylene and polymer repeat units of eitherhexafluoropropylene or pentafluoropropylene. Yet another composition ofthe invention contains polymer repeat units of vinylidene fluoride,polymer repeat units of perfluoroalkyl perfluorovinyl ether, and polymerrepeat units of tetrafluoroethylene, optionally also containing repeatunits of hexafluoropropylene.

In particular, and for the above described embodiments, component (b)(1)may contain 30-65 weight percent, preferably 30-60 weight percentvinylidene fluoride units; 20-45 weight percent, preferably 25-40 weightpercent hexafluoropropylene units; and 0-35 weight percent, preferably10-30 weight percent tetrafluoroethylene units. Alternatively, (b)(1)can also be composed of 15-65 weight percent, preferably 25-60 weightpercent of vinylidene fluoride units; 0-55 weight percent, preferably5-40 weight percent tetrafluoroethylene units; and, 25-45 weight percentpreferably 30-45 weight percent, of perfluoroalkyl perfluorovinyl etherunits having the formula CF₂ ═CFO(CF₂ CFXO)_(n) R_(f), where X is F ortrifluoromethyl, n is 0-5 and R_(f) is a perfluoroalkyl group of 1-6carbon atoms. A preferred perfluoroalkyl perfluorovinyl ether isperfluoro(methyl vinyl ether), hereinafter referred to as PMVE.Alternatively, PMVE can be used in admixture with another perfluoralkylperfluorovinyl ether, so long as the total perfluoroalkyl perfluorovinylether content is in the range of 15-35 mole percent in the polymer.

In useful embodiments, component (b)(2) is composed of 10-40 molepercent, preferably 20-40 mole percent ethylene units; 32-60 molepercent tetrafluoroethylene units; and, 20-40 mole percent, preferably20-35 mole percent of perfluoroalkyl perfluorovinyl ether units havingthe formula CF₂ ═CFO(CF₂ CFXO)_(n) R_(f), where X is F ortrifluoromethyl, n is 0-5 and R_(f) is a perfluoroalkyl group of 1-6carbon atoms. A preferred perfluoroalkyl perfluorovinyl ether is PMVE.Alternatively, PMVE can be used in admixture with another perfluoroalkylperfluorovinyl ether, so long as the total perfluoroalkyl perfluorovinylether content is in the range of 15-35 mole percent in thefluoroelastomer. In U.S. Pat. No. 4,694,045 a variety of perfluoro alkylperfluorovinyl ethers are disclosed, and are incorporated herein byreference.

The elastomers described herein are prepared by free radical emulsionpolymerization in a continuous stirred tank reactor. Polymerizationtemperatures may be in the range 40° to 130° C., preferably 70° to 115°C., at pressures of 2 to 8 MPa and residence times of 10 to 240 minutes.A residence time of 20 to 60 minutes is preferred for vinylidenefluoride copolymers. Free radical generation is effected using awater-soluble initiator such as ammonium persulfate, either by thermaldecomposition or by reaction with a reducing agent such as sodiumsulfite. Initiator levels are set low enough so that iodine end groupspredominate over those from initiator fragments. This leads to thedesired low polymer viscosity and contributes to good flowcharacteristics and good vulcanizate properties, including compressionset resistance. The polymer dispersion is stabilized with an inertsurface-active agent such as ammonium perfluorooctanoate, usually withaddition of a base such as sodium hydroxide or a buffer such as disodiumphosphate to control pH in the range 3 to 7. After polymerization,unreacted monomer is removed from the reactor effluent latex byvaporization at reduced pressure. The polymer is recovered from latex bycoagulation, e.g., by reducing pH to about 3 by acid addition and addinga salt solution such as calcium nitrate, magnesium sulfate, or potassiumaluminum sulfate in water, followed by separation of serum from polymer,washing with water, and drying of the wet polymer.

Fluoroelastomers made by the method described are generally cured by afree radical process. A curable composition comprises polymer and aperoxide to generate free radicals at curing temperatures. A dialkylperoxide which decomposes at a temperature above 50° C. is especiallypreferred when the composition is to be processed at elevatedtemperatures before it is cured. In many cases one will prefer to use adi-tertiarybutyl peroxide having a tertiary carbon atom attached toperoxy oxygen. Among the most useful peroxides of this type are2,5-dimethyl-2,5-di(tertiarybutylperoxy) hexyne-3 and2,5-dimethyl-2,5-di(tertiarybutylperoxy) hexane. Other peroxides can beselected from such compounds as dicumyl peroxide, dibenzoyl peroxide,tertiarybutyl perbenzoate, anddi[1,3-dimethyl-3-(t-butylperoxy)butyl]carbonate.

Another material which is usually blended with the composition before itis made into end products is a coagent composed of a polyunsaturatedcompound which is capable of cooperating with the peroxide to provide auseful cure. These crosslinking coagents can be added in an amount equalto 0.5-10 percent, preferably about 1-7 percent, by weight of thecopolymer content, and may be one or more of the following compounds:triallyl cyanurate; triallyl isocyanurate; tri(methallyl) isocyanurate;tris(diallylamine)-s-triazine; triallyl phosphite; N,N-diallylacrylamide; hexaallyl phosphoramide; N,N,N',N'-tetraallyltetraphthalamide; N,N,N',N'-tetraallyl malonamide; trivinylisocyanurate; 2,4,6-trivinyl methyltrisiloxane; andtri(5-norbornene-2-methylene) cyanurate. Particularly useful is triallylisocyanurate, hereafter called TAIC.

Optionally, at least one metal compound selected from divalent metaloxides or divalent metal hydroxides is frequently blended with thefluoroelastomer during preparation or before it is cured. The presenceof such compounds improves the heat aging resistance and thermalstability of the polymer. Representative metal compounds include theoxides and hydroxides of magnesium, zinc, calcium or lead. A metal saltof a weak acid can also be used along with the oxide and/or hydroxide.Representative salts of weak acids include the barium-, sodium-,potassium-, lead-, and calcium-/-stearate, -benzoate, -carbonate,-oxalate and -phosphite. Magnesium and lead oxides are especiallypreferred. The metal compound is added to the fluoroelastomer in anamount equal to 1-15 weight percent, preferably 2-10 weight percent,based on fluoroelastomer.

The fluoroelastomers can also contain conventional fillers such ascarbon black, clay, silica and talc. Other fillers, pigments,antioxidants, stabilizers and the like can also be used. It isparticularly advantageous to add carbon black to the fluoroelastomer toincrease its modulus. Usually amounts of from 5-50 parts per hundredparts of fluoroelastomer are used, with the particular amount determinedfrom the particle size of the carbon black and the desired hardness ofthe cured composition.

As described hereinabove, the fluoroelastomer compositions of thisinvention have a unique structure in which free radical-reactivebromo-sites are randomly attached along the polymer chain and iodinatedsites are at the chain ends. Thus, when this fluoroelastomer iscrosslinked by the action of organic peroxides and crosslinkingcoagents, products are obtained having enhanced strength, compressionset and ease of processibility that are useful in the preparation ofinjection molded shaft seals, gaskets and other molded parts.

The subject invention will be more fully appreciated with reference tothe following examples.

EXAMPLES EXAMPLE 1

A continuous emulsion polymerization was carried out in a well-stirred4.0-liter stainless steel reaction vessel. The reactor was filled withan aqueous solution containing 0.48 g ammonium persulfate (APS), 0.30 gsodium hydroxide, and 0.75 g ammonium perfluorooctanoate (FC-143) soapper liter of deionized water. The reactor was heated to 110° C. and theaqueous solution was fed at 6.0 L/h. The reactor was kept liquid-full at6.2 MPa by means of a back-pressure control valve in the effluent line.After 30 minutes the reaction was started by introducing a gaseousmonomer mixture consisting of 315 g/h tetrafluoroethylene (TFE), 389 g/hvinylidene fluoride (VF₂), and 644 g/h of hexafluoropropylene (HFP) fedthrough a diaphragm compressor. After 15 minutes a feed was establishedof a mixture of 6.9 g/h BTFB and 3.4 g/h of 1,4-diiodoperfluorobutane(total solution feed 5.6 mL/h). After 2 hours, effluent dispersion wascollected for 8.5 hours.

The effluent polymer dispersion was separated from residual monomers ina degassing vessel at atmospheric pressure. The dispersion had pH=4.0and contained 15.6 weight percent solids. The fluoroelastomer wasisolated from the dispersion by reducing pH to about 3 with dilutesulfuric acid and coagulating with potassium aluminum sulfate solution.The coagulated polymer was allowed to settle, supernatant serum wasremoved, and the polymer was washed by reslurrying in water twice beforefiltering. The wet crumb was dried in an air oven at 50°-60° C. to amoisture content of less than 1%.

About 9 kg of polymer was recovered at an overall conversion of 81%. Thepolymer had the composition 28% TFE, 34% VF₂, 37% HFP, and 0.6% BTFB,and also contained 0.15% iodine, corresponding to about 90% of that fedin diiodoperfluorobutane. The polymer was an amorphous fluoroelastomerwith glass transition temperature -9° C., as determined by differentialscanning calorimetry (heating mode, 10° C./min, onset of transition).Fluoroelastomer inherent viscosity was 0.36 dL/g, measured at 30° C. inmethyl ethyl ketone, and Mooney viscosity was measured as ML-10 (100°C.)=56.

The number average molecular weight, M_(n), was determined to be 68,100Daltons by membrane osmometry measurements in methyl ethyl ketone. Fromthis information and the analyses for bromine and iodine, it wascalculated that the average polymer chain contained 2.1 bromine atomsper chain and 0.9 iodine atoms per chain. The ratio of weight average tonumber average molecular weight, M_(w) /M_(n) was estimated frommolecular weight determinations in dimethylacetamide solvent at 135° C.using a Waters Associates Gel Permeation Chromatograph. Data are shownin Table I.

A curable fluoroelastomer composition was prepared by mixing thefollowing ingredients on a two-roll rubber mill whose rolls were heatedto about 25° C.: 100 parts fluoroelastomer of Example 1, 30 parts MT(N990) carbon black, 3 parts Maglite Y magnesium oxide, 3 parts TAIC,and 3 parts "Luperco" 101-XL peroxide (45% of2,5-dimethyl-2,5-di(t-butyl peroxy)hexane and 55% inert filler). Curecharacteristics of the composition were measured with an oscillatingdisk rheometer (ODR) at a cure time of 12 minutes at 177° C., accordingto ASTM D-2084 (1 deg. arc). The time required to reach a torqueincrease (from the minimum) of 0.2 Joule, t_(s) (0.2), was 1.4 minutesand the time required to reach 90% of the cure state reached in 12minutes was determined as t'90=3.8 minutes. Test samples werepress-cured for 10 minutes at 177° C. and post-cured in a circulatingair oven for 24 hours at 200° C. Stress-strain properties weredetermined according to ASTM D-412 as: 100% modulus, M₁₀₀ =4.5 MPa;tensile at break, T_(B) =15.9 MPa; elongation at break, E_(B) =260%.Compression set, measured on pellets in air, was 67% after 70 hours at200° C. The results are reported in Tables I and II.

EXAMPLES 2 AND 3

Fluoroelastomers 2 and 3 were made by continuous emulsion polymerizationin a well-stirred 4.0-liter reaction vessel, as in Example 1, and arefurther described in Table I. Properties of the cured fluoroelastomersare shown in Table II.

COMPARATIVE EXAMPLE A

A fluoroelastomer was prepared as in Examples 1-3, except that BTFB wasnot fed to the reaction mixture, giving a fluoroelastomer characterizedas shown in Table I and having the vulcanizate properties shown in TableII.

EXAMPLES 4-6

Elastomeric copolymers of TFE, VF2, PMVE, and BTFB were prepared bycontinuous emulsion polymerizations as in Example 1, with PMVE monomerused in place of HFP. Startup and general operation were as described inprevious examples. Polymerization conditions and characterization of thebromine- and iodine-containing TFE/VF2/PMVE fluoroelastomers are shownin Table III. Properties of the cured fluoroelastomers are shown inTable IV.

COMPARATIVE EXAMPLE B

A fluoroelastomer was prepared as in Examples 4-6, except that isopropylalcohol was used as modifier in place of diiodoperfluorobutane, giving afluoroelastomer characterized as shown in Table III and having thevulcanizate properties shown in Table IV.

EXAMPLE 7

An elastomeric copolymer of ethylene, TFE, PMVE, and BTFB was made bycontinuous emulsion polymerization at 90° C. as in Examples 1-3. Startupand general operation were as described in previous examples. Aqueoussolution was fed to a 4-liter reactor at 1.2 L/h to maintain solutefeeds of 1.13 g/h APS initiator, 12 g/h disodium phosphate heptahydrate,and 7.0 g/h FC-143 soap. Gaseous monomers were fed at 30 g/h ethylene,178 g/h TFE, and 257 g/h PMVE. BTFB curesite monomer was fed at 2.6 g/hand 1,4-diiodoperfluorobutane at 1.6 g/h in tertiary-butanol solution.After 4 hours equilibration, effluent dispersion was collected for 8.5hours. The dispersion had pH=6.5 and contained 23% solids. Dispersion pHwas adjusted to about 3 by addition of dilute nitric acid and thepolymer was coagulated by adding calcium nitrate solution, then washedand dried as in Example 1. About 3.4 kg polymer was recovered at overallconversion 72%. Polymer composition was 8.7% ethylene, 46% TFE, 44%PMVE, and 0.7% BTFB, and contained 0.16% iodine.

A curable fluoroelastomer composition was prepared by mixing thefollowing ingredients on a two-roll rubber mill whose rolls were heatedto about 25° C. 100 parts fluoroelastomer, 30 parts MT (N990) black, 3parts litharge, 3 parts triallyl isocyanurate (TAIC), and 3 parts"Luperco" 101-XL peroxide (45% of 1,5-dimethyl-1,5-di(t- butylperoxy)hexane and 55% inert filler). Cure characteristics of thecomposition were measured with an oscillating disk rheometer (ODR) at acure time of 12 minutes at 177° C., according to ASTM D-2084, and hadthe following values: M_(L) =0.45 joule, M_(H) =3.05 joules, t_(s)(0.2)=1.4 min and t'90=7.2 min. Test samples were press-cured for 15 minat 177° C. and post-cured in a circulating air oven for 24 hrs at 232°C. Stress-strain properties were determined according to ASTM D-412 as:100% modulus, M100= 5.0 MPa, TB=13.1 MPa and elongation at break,EB=220%. Compression set, measured on pellets in air, was 58% after 70hrs at 200° C.

                  TABLE I                                                         ______________________________________                                                                             Compara-                                                                      tive                                     Example        1       2       3     A                                        ______________________________________                                        Aq. sol'n, L/h 6       4       4     4                                        APS, g/h       2.87    1.6     1.16  1.6                                      NaOH, g/h      1.8     1.2     0.8   1.2                                      FC-143, g/h    4.5     4       4     4                                        I(CF.sub.2).sub.4 I, g/h                                                                     3.4     3.8     4.4   3.8                                      Monomer feed, g/h                                                             TFE            315     242     217   254                                      VF2            389     299     268   316                                      HFP            644     489     437   523                                      BTFB           6.9     5.3     4.8   --                                       Dispersion                                                                    % Solids       15.6    17.8    15.7  18.6                                     pH             4.0     3.9     3.2   4.1                                      Pol. rate, g/h 1097    857     738   906                                      Conversion, %  81      82      79    83                                       Polymer Composition,                                                          wt. %                                                                         VF2            34      34      35    34                                       TFE            28      27      28    27                                       HFP            37      38      36    38                                       BTFB           0.63    0.62    0.65  --                                       % Br           0.24    0.24    0.25  0                                        % I            0.15    0.2     0.25  0.21                                     Viscosity                                                                     ML-10 (100° C.)                                                                       56      54      37    52                                       Inh. Visc.     0.36    0.36    0.36  0.34                                     M.sub.n by Osmometry, Dalton                                                                 68,100  76,900  66,300                                                                              70,000                                   M.sub.w /M.sub.n, by GPC                                                                     2.1     1.9     1.9   1.8                                      Br/chain       2.1     2.3     2.1   0                                        I/chain        0.9     1.5     1.8   1.2                                      DSC: Tg, °C.                                                                          -9      -10     -13   -8                                       ______________________________________                                    

                  TABLE II                                                        ______________________________________                                                                            Comparative                               Example          1      2      3    A                                         ______________________________________                                        Recipe, phr                                                                   MT black         30     30     30   30                                        Maglite Y        3      3      3    3                                         Peroxide         3      3      3    3                                         TAIC             3      3      3    3                                         ODR, 177° C.                                                           ML, joules       0.34   0.57   0.34 0.57                                      MH, joules       2.83   5.42   5.99 5.31                                      ts(0.2), min     1.4    1.2    1.2  1.2                                       t'90, min        3.8    3.5    3.0  2.5                                       Press Cure, min/177° C.                                                                 10     10     10   10                                        Post Cure 24 hrs, Temp., °C.                                                            200    200    200  200                                       Stress-strain                                                                 M100, MPa        4.5    6.4    7.8  5.8                                       TB, MPa          15.9   17.6   18.5 17.6                                      EB, %            260    200    180  230                                       Compression Set, (%) Pellets,                                                                  67     46     38   59                                        70h/200° C.                                                            ______________________________________                                    

                  TABLE III                                                       ______________________________________                                                                             Compara-                                                                      tive                                     Example        4       5       6     B                                        ______________________________________                                        Aq. sol'n, L/h 6       6       6     6                                        APS, g/h       2.12    2.12    2.21  3.77                                     NaOH, g/h      0.8     0.8     1     2.0                                      FC-143, g/h    4       4       4     4                                        I(CF.sub.2).sub.4 I, g/h                                                                     8.4     5.6     4.2   0                                        Isopropyl alchohol,                                                                          --      --      --    1.35                                     g/h                                                                           Monomer feed, g/h                                                             TFE            341     365     363   407                                      VF2            497     496     504   570                                      PMVE           655     645     647   695                                      BTFB           11.3    11.3    16.9  18.0                                     Dispersion                                                                    % Solids       18.7    18.5    18.7  21.1                                     pH             3.6     3.3     4.0   4.7                                      Pol. rate, g/h 1377    1354    1373  1589                                     Conversion, %  91      89      89    94                                       Polymer Composition,                                                          wt. %                                                                         VF2            35      35      35    35                                       TFE            24      26      25    25                                       PMVE           40      38      38    39                                       BTFB           0.82    0.83    1.23  1.13                                     % Br           0.32    0.32    0.47  0.44                                     % I            0.30    0.22    0.15  0                                        Viscosity                                                                     ML-10 (100° C.)                                                                       18      32      50    74                                       Inh. Visc.     0.39    0.43    0.47  0.57                                     M.sub.n by Osmometry,                                                                        57,200  70,000  87,000                                                                              --                                       Daltons                                                                       M.sub.w /M.sub.n, by GPC                                                                     1.8     2.0      2.2  --                                       Br/chain       2.3     2.8     5.2   --                                       I/chain        1.5     1.3     1.2   --                                       DSC: Tg, °C.                                                                          -27     -27     -26   -23                                      ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                                                            Comparative                               Example         4      5       6    B                                         ______________________________________                                        Recipe, phr                                                                   MT black        30     10      30   30                                        Nyad 400 (calcium                                                             silicate)       --     25      --   --                                        Maglite Y       3      3       3    3                                         Peroxide        1.5    1.5     1.5  3                                         TAIC            1.5    1.5     1.5  1.5                                       ODR, l77° C.                                                           ML, joules      0.15   --      0.66 0.99                                      MH, joules      5.31   --      5.31 3.26                                      ts(0.2), min    1.3    --      1.1  1.4                                       t'90, min       3.4    --      3.8  6.1                                       Press Cure, min 10     6       10   10                                        at l77° C.                                                             Post Cure 24 hrs.                                                                             200    200     200  200                                       Temp., °C.:                                                            Stress-strain                                                                 M100, Mpa       5.2    4.7     5.6  3.1                                       TB, MPa         14.0   11.2    13.4 10.8                                      EB, %           180    250     160  250                                       Compression Set, Pellets,                                                                     33     --      33   57                                        70h/200° C.                                                            ______________________________________                                    

I claim:
 1. A composition comprising a peroxide curable fluoroelastomerprepared by continuous emulsion polymerization in the presence ofiodinated compounds of the formula RI_(n), where R is a radical of 1-8carbon atoms and selected from the group consisting of fluorocarbons andchlorofluorocarbons, I is iodine, and n is 1 or 2, said iodine beingbonded at a substantial number of terminal positions on thefluoroelastomer, the amount of iodinated compound being sufficient toprovide at least 0.1 weight percent iodine in the fluoroelastomer,comprising:(a) up to 3 weight percent, based on the total weight ofcomponents (a) and (b), of polymer repeat units selected from the groupconsisting of fluoroolefins and fluorovinyl ethers, said unitscontaining bromine and being present in an amount present to provide0.1-1.0 weight percent bromine in the fluoroelastomer; and (b)complementally, at least 97 weight percent, based on the total weight ofcomponents (a) and (b), of: (1) polymer repeat units of vinylidenefluoride, and polymer repeat units of one or more fluoroolefinscopolymerizable therewith, said fluoroolefins containing 2-8 carbonatoms and at least as many fluorine atoms as carbon atoms, optionally,polymer repeat units provided by or (2) 32-60 mole percent of polymerrepeat units of tetrafluoroethylene, 20-40 mole percent of polymerrepeat units of perfluoroalkyl perfluorovinyl ethers, and 10-40 molepercent of polymer repeat units of ethylene.
 2. The composition of claim1 wherein the fluoroelastomer contains from 0.1 to 0.5 weight percentiodine.
 3. The composition of claim 1 wherein the fluoroelastomercontains from 0.15 to 0.6 weight percent bromine.
 4. The composition ofclaim 1 wherein the fluoroolefin of (a) is4-bromo-3,3,4,4-tetrafluorobutene-1.
 5. The composition of claim 1wherein the fluorovinyl ether of (a) has the formula CF₂Br--R--O--CF═CF₂ or ROCF═CFBr, where R is a lower alkyl group orfluoroalkyl group.
 6. The composition of claim 1 wherein (b)(1) consistsof polymer repeat units of vinylidene fluoride and polymer repeat unitsof a fluoroolefin selected from the group consisting ofhexafluoropropylene and pentafluoropropylene.
 7. The composition ofclaim 1 wherein (b)(1) consists of polymer repeat units of vinylidenefluoride, polymer repeat units of tetrafluoroethylene, and polymerrepeat units of fluoroolefins selected from the group consisting ofhexafluoropropylene and pentafluoropropylene.
 8. The composition ofclaim 7 wherein (b)(1) is comprised of 30-65 weight percent of polymerrepeat units of vinylidene fluoride, 0-35 weight percent of polymerrepeat units of tetrafluoroethylene, and 20-45 weight percent of polymerrepeat units of hexafluoropropylene.
 9. The composition of claim 7wherein (b)(1) is comprised of 30-60 weight percent of polymer repeatunits of vinylidene fluoride, 10-30 weight percent of polymer repeatunits of tetrafluoroethylene, and 25-40 weight percent of polymer repeatunits of hexafluoropropylene.
 10. The composition of claim 1 wherein(b)(1) consists of polymer repeat units of vinylidene fluoride, polymerrepeat units of perfluoroalkyl perfluorovinyl ether, polymer repeatunits of tetrafluoroethylene, and optionally, polymer repeat units ofhexafluoropropylene.
 11. The composition of claim 10 wherein (b)(1) iscomprised of 15-65 weight percent of polymer repeat units of vinylidenefluoride, 0-55 weight percent of polymer repeat units oftetrafluoroethylene, and 25-45 weight percent of polymer repeat units ofperfluoroalkyl perfluorovinyl ether.
 12. The composition of claim 10wherein (b)(1) is comprised of 25-60 weight percent of polymer repeatunits of vinylidene fluoride, 5-40 weight percent of polymer repeatunits of tetrafluoroethylene, and 30-45 weight percent of polymer repeatunits of perfluoroalkyl perfluorovinyl ether.
 13. The composition ofclaim 1 wherein (b)(1) contains units of a perfluoroalkyl perfluorovinylether of the formula CF₂ ═CFO(CF₂ CFXO)_(n) R_(f), where X is F ortrifluoromethyl, n is 0-5 and R_(f) is a perfluoroalkyl group of 1-6carbon atoms.
 14. The composition of claim 13 wherein the perfluoroalkylperfluorovinyl ether is perfluoro(methyl vinyl ether).
 15. Thecomposition of claim 1 wherein (b)(1) contains two perfluoroalkylperfluorovinyl ethers, where one ether is perfluoro(methyl vinyl ether),and the total content of the perfluoroalkyl perfluorovinyl ethers is15-35 mole percent in the fluoroelastomer.
 16. The composition of claim1 wherein component (b)(2) consists of 32-60 mole percent of polymerrepeat units of tetrafluorethylene, 20-40 mole percent of polymer repeatunits of perfluroalkyl perfluorovinyl ether, and 10-40 mole percent ofpolymer repeat units of ethylene.
 17. The composition of claim 1 wherein(b)(2) consists of 32-60 mole percent of polymer repeat units oftetrafluoroethylene, 20-35 mole percent of polymer repeat units ofperfluoroalkyl perfluorovinyl ether, and 20-40 mole percent of polymerrepeat units of ethylene.
 18. The composition of claim 1 wherein theperfluoroalkyl perfluorovinyl ether of (b)(2) has the formula CF₂═CFO(CF₂ CFXO)_(n) R_(f), where X is F or trifluoromethyl, n is 0-5 andR_(f) is a perfluoroalkyl group of 1-6 carbon atoms.
 19. The compositionof claim 18 wherein the perfluoroalkyl perfluorovinyl ether isperfluoro(methyl vinyl ether).
 20. The composition of claim 1 wherein(b)(2) contains two perfluoroalkyl perfluorovinyl ethers, where oneether is perfluoro(methylvinyl ether), and the total content of theperfluoroalkyl perfluorovinyl ethers is 15-35 mole percent in thefluoroelastomer.
 21. The composition of claim 1 wherein the iodinatedcompound is a perhalogenated diiodide.
 22. The composition of claim 21wherein the perhalogenated diiodide is selected from the groupconsisting of 1,3-diiodoperfluoro-n-propane,1,4-diiodoperfluoro-n-butane, 1,3-diiodo-2-chloroperfluoro-n-propane,and 1,5-diiodo-2,4-dichloro-perfluoro-n-pentane.